1. Field of the Invention
The present invention relates in general to semi-conductor devices, and more particularly to a high voltage generating circuit for semi-conductor devices for removing a threshold voltage loss in clamping and charge transfer devices to increase an efficiency of the semi-conductor device.
2. Description of the Prior Art
In a semi-conductor device of the type called a complementary MOS (CMOS), there has sometimes been needed a high voltage above a source voltage Vcc or a negative voltage below a ground voltage Vss.
For example, the high voltage above the source voltage Vcc is mainly used to overcome a threshold voltage loss being present in n-channel MOSFETs in the transfer of a logic high state, such as a word line potential for perfect transfer of the source voltage Vcc corresponding to storage data "1 " into a memory cell in a dynamic RAM (DRAM) or a drive potential for driving of a pull-up gate using n-channel MOSFETs in an output buffer; the negative voltage below the ground voltage Vss is used as a bias voltage of a p-type substrate.
With reference to FIG. 1, there is shown a circuit diagram of a conventional high voltage generating circuit for semi-conductor devices. The illustrated circuit comprises a ring oscillator 1 as a oscillation signal generator, a n-channel MOSFET M3 including its common source-drain stage coupled to an oscillation signal .phi.osc1 generated from the ring oscillator 1, for functioning as a charge pump, a n-channel MOSFET M1 including its drain and gate, both coupled to a source voltage Vcc and its source connected to the gate of the n-channel MOSFET M3, for clamping-functioning, and a diode-type n-channel MOSFET M2 including its drain and gate, both connected to the gate of the n-channel MOSFET M3.
The operation of the conventional high voltage generating circuit for semi-conductor devices with the above-mentioned construction will be described with reference to FIG. 2.
The ring oscillator 1 generates the oscillation signal .phi.osc1 oscillating with a constant period Tosc and a magnitude of amplitude of the source voltage Vcc. When the oscillation signal .phi.osc1 is at the ground voltage Vss level, a node 12 at the gate stage of the n-channel MOSFET M3 is charged with a voltage Vcc-Vt resulting from a threshold voltage loss on the source voltage Vcc through the n-channel MOSFET M1. If the oscillation signal .phi.osc1 is transited from the ground voltage Vss level to the source voltage Vcc level, a voltage V2 at the node 12 is transited, according to the coupling effect, from a voltage Vcc-Vth level lower by a threshold voltage Vth than the source voltage Vcc to a voltage V2,M level above the source voltage Vcc, because of impossibility of instant variation of voltages at the common source-drain stage and the gate stage of the n-channel MOSFET M3. Then, the voltage V2,M at the node 12 is transferred to an output stage Vpp by turning on of the diode-type n-channel MOSFET M2, the drain and the gate of which are connected to the node 12, and hence is charged into a load capacitor CL.
Through the repetition of the oscillating operation as mentioned above is prevented a leakage current through a load resistor RL, and thus the potential at the output stage Vpp can rise, but the threshold voltage loss on the n-channel MOSFETs M1 and M2 causes the operation efficiency to be not high.
That is, the charged potential at the node 12 is maintained at a voltage Vcc-Vt1 level lower by a threshold voltage Vt1 on the n-channel MOSFET M1 than the source voltage Vcc due to the threshold voltage loss on the n-channel MOSFET M1, and the high voltage applied to the node 12 by the operation of the n-channel MOSFET M3 is transferred to the output stage Vpp, with suffering a threshold voltage Vt2 loss on the n-channel MOSFET M2. As a result, the maximum potential at the output stage Vpp is at a voltage V2,M-Vt2 level lower by the threshold voltage Vt2 on the n-channel MOSFET M2 than the voltage V2,M.
For the purpose of the improvement in the above-stated problem, there has been proposed a high voltage generating circuit with a cross-coupled charge pump.
With reference to FIG. 3, there is shown a circuit diagram of the conventional high voltage generating circuit with the cross-coupled charge pump. The illustrated circuit comprises a n-channel MOSFET M11 including its common source-drain stage coupled to a first oscillation signal .phi.osc11 generated from a ring oscillator, for functioning as a charge pump, a n-channel MOSFET M12 including its common source-drain stage coupled to a second oscillation signal .phi.osc12 generated from the ring oscillator, for functioning as a charge pump, a pair of n-channel MOSFETs M13 and M14 including their sources connected respectively to gates of the n-channel MOSFETs M11 and M12, their drains coupled to a source voltage Vcc and their gates cross-coupled to their sources, for clamping-functioning, and a pair of diode-type n-channel MOSFETs M15 and M16 including their drains and gates, both connected respectively to the gates of the n-channel MOSFETs M11 and M12. Herein, the first and second oscillation signals .phi.osc11 and .phi.osc12 each has a magnitude of amplitude of the source voltage Vcc and a phase difference of 180.degree. with respect to each other.
The operation of the conventional high voltage generating circuit with the cross-coupled charge pump constructed as stated above will be described with reference to FIG. 4.
If the first oscillation signal .phi.osc11 is at the source voltage Vcc level and the second oscillation signal .phi.osc12 is at the ground voltage Vss level, a node 33 at the gate stage of the capacitor-type n-channel MOSFET M11 is at a voltage level above the source voltage Vcc, thereby allowing cross-coupled n-channel MOSFET M14 to be turned on. As a result, the turning on of the n-channel MOSFET M14 allows a node 34 at the gate stage of the capacitor-type n-channel MOSFET M12 to be charged with the source voltage Vcc with no threshold voltage loss and hence the capacitor-type n-channel MOSFET M12 to be charged with similarly.
When the first oscillation signal .phi.osc11 is transited from the source voltage Vcc level to the ground voltage Vss level and the second oscillation signal .phi.osc12 is transited from the ground voltage Vss level to the source voltage Vcc level, the node 34 at the gate stage of the capacitor-type n-channel MOSFET M12 is at a voltage level above the source voltage Vcc by the operation of the n-channel MOSFET M12, thereby allowing the cross-coupled n-channel MOSFET M13 to be turned on. As a result, the turning on of the n-channel MOSFET M13 allows the node 33 at the gate stage of the capacitor-type n-channel MOSFET M11 to be charged with the source voltage Vcc with no threshold voltage loss and hence the capacitor-type n-channel MOSFET M11 to be charged with similarly.
Therefore, the potential at the node 34 is transited from the source voltage Vcc level to a voltage Vcc+V level above the source voltage Vcc and the diode-type n-channel MOSFET M16, the drain and the gate of which are connected to the node 34, is turned on, thereby allowing the voltage Vcc+V at the node 34 to be transferred to an output stage Vpp and to be charged into a load capacitor CL.
Therefore, the high voltage generating circuit with the cross-coupled charge pump as mentioned above is capable of removing the threshold voltage loss involved in the conventional diode-type clamping device and redoubling a duty cycle on time in comparison with the conventional high voltage generating circuit by means of two oscillation signals with a phase difference of 180.degree. and capacitors M11 and M12 activated by the oscillation signals.
However, the conventional high voltage generating circuit with the cross-coupled charge pump has a disadvantage, in that there is present a threshold voltage loss on the diode-type n-channel MOSFETs M15 and M16 at the output stage.